Dew point determination

The practical measurement of water activity nowadays is usually performed by dew point determination or a change of electrical properties of the material depending on the relative humidity. 

Either the temperatures or flows could be adjusted first. The common choice is to correct the temperature. Correction of temperatures is usually done through either bubble-point or dew-point determinations on the calculated stage compositions. After correcting the stage temperatures, the liquid and vapor enthalpies may be obtained from the calculated compositions, and the flows corrected by solution of the now linear heat balance equations of Table I. 

The Basic Protocol describes the determination of water activity of a product using a chilled mirror dew-point water activity meter. Dew point is a primary measurement of vapor pressure that has been in use for decades (Harris, 1995). Dew-point instruments are accurate, fast, simple to use, and precise (Richard and Labuza, 1990 Snavely et al., 1990 Roa and Tapia de Daza, 1991). In a dew-point instrument, water activity is measured by equilibrating the liquid-phase water in the food sample with the vapor-phase water in the headspace, and then measuring the vapor pressure of the headspace. The basic principle involved in dew-point determinations of vapor pressure in air is that air may be cooled without change in water content until it saturates. The dew-point temperature is the temperature at which the air reaches saturation. It is determined in practice by measuring... 

Detergent binding capacity, determination of protein hydrophobicity, 304-305 Dew-point determination method, water activity... 

The computer subroutines for calculation of vapor-liquid equilibrium separations, including determination of bubble-point and dew-point temperatures and pressures, are described and listed in this Appendix. These are source routines written in American National Standard FORTRAN (FORTRAN IV), ANSI X3.9-1978, and, as such, should be compatible with most computer systems with FORTRAN IV compilers. Approximate storage requirements for these subroutines are given in Appendix J their execution times are strongly dependent on the separations being calculated but can be estimated (CDC 6400) from the times given for the thermodynamic subroutines they call (essentially all computation effort is in these thermodynamic subroutines). 

Once the bubble point is reached (at point B), the first bubble of ethane vapour is released. From point B to C liquid and gas co-exist in the cell, and the pressure is maintained constant as more of the liquid changes to the gaseous state. The system exhibits infinite compressibility until the last drop of liquid is left In the cell (point C), which is the dew point. Below the dew point pressure only gas remains in the cell, and as pressure is reduced below the dew point, the volume increase is determined by the compressibility of the gas. The gas compressibility is much greater than the liquid compressibility, and hence the change of volume for a given reduction in pressure (the... 

The experiment could be repeated at a number of different temperatures and initial pressures to determine the shape of the two-phase envelope defined by the bubble point line and the dew point line. These two lines meet at the critical point, where it is no longer possible to distinguish between a compressed gas and a liquid. 

Example 3 Air Heating Air is heated by a steam coil from 30 F dry-bulb temperature and 80 percent relative humidity to 75 F dry-bulb temperature. Find the relative humidity, wet-bulb temperature, and dew point of the heated air. Determine the quantity of heat added per pound of dry air. 

Relative humidity and dew point can be determined for other than atmospheric pressure from the partial pressure of water in the mixture and from the vapor pressure of water vapor. The partial pressure of water is calculated, if ideal-gas behavior is assumed, as... 

Example 8 Determination of Air Properties For a barometric pressure of 25.92 inHg (Ap = —4), a dry-bulb temperature of 90 F, and a wet-bulb temperature of 70 F determine the following absolute humidity, enthalpy, dew point, relative humidity, and specific volume. 

Although the dew-point method may be considered a fundamental technique for determining humidity several uncertainties occur in its use. It is not always possible to measure precisely the temperature of the polished surface or to eliminate gradients across the surface. It is also difficult to detect the appearance or disappearance of fog the usual practice is to take the dew point as the average of the temperatures when fog first appears on cooling and disappears on heating. 

A third fundamental type of laboratory distillation, which is the most tedious to perform of the three types of laboratory distillations, is equilibrium-flash distillation (EFV), for which no standard test exists. The sample is heated in such a manner that the total vapor produced remains in contact with the total remaining liquid until the desired temperature is reached at a set pressure. The volume percent vaporized at these conditions is recorded. To determine the complete flash curve, a series of runs at a fixed pressure is conducted over a range of temperature sufficient to cover the range of vaporization from 0 to 100 percent. As seen in Fig. 13-84, the component separation achieved by an EFV distillation is much less than by the ASTM or TBP distillation tests. The initial and final EFN- points are the bubble point and the dew point respectively of the sample. If desired, EFN- curves can be established at a series of pressures. 

When cooling combustion flue gas for heat recovery and efficiency gain, the temperature must not be allowed to drop below the sulfur trioxide dew point. Below the SO3 dew point, very corrosive sulfuric acid forms. The graph in Figure 1 allows determination of the acid dew point us shown in Example 1. 

The elemental concentration of S, expressed as 1.7 vol.%, in Table 1, is used to enter Figure 1. One then proceeds to the elemental concentration of carbon as determined from Table 1, which is 0.891. Assuming 10% excess air and proceeding upward to the 0.891 carbon line and then to the left gives 152°C dew point. 

Saturated volume is the volume in cubic feet of 1 lb of dry air when it is saturated with water vapor that is, it is the humid volume at saturation, and is determined by the temperature and pressure. The humid volume of air equals the product of its saturated volume at its dew point and the ratio of the absolute temperature of the air to the absolute temperature of its dew point. 

CHEMCALC 1, Separations Calculations Gulf Publishing Company, Book Division P.O. Box 2608 Houston, TX 77252 Programs for use with multi-component mixtures to determine the conditions and compositions at the dew point and at the bubble point. 

The analyses of gases in the oil industry comprises the determination of the inert gases (He, Hj, O2, Ar and N2), low-boiling compounds (CO, CO2, H2S, COS) and the lower hydrocarbons, saturated and unsaturated, up to hexane. Some special samples. Such as natural gas, have to be analysed for low concentrations of higher-boiling compounds (up to CiqS) since such compounds have an important influence on the calorific value and dew point. 

The economic value of natural gas is primarily determined by the thermal energy it contains, which is expressed in British thermal units (Btu) or calorific value (CV). Other important physical properties comprise the liquid content, the burning characteristics, the dew point and the compressibility. In order to enable the calculation of these properties from its composition, a natural gas analysis should contain a detailed determination of all of the individual components, even in the low-concentration range. 

The direct-solution method of Akers and Wade [1] is among several which attempt to reduce the amount of trial-and-error solutions. This has been accomplished and has proven quite versatile in application. The adaptation outlined modifies the symbols and rearranges some terms for convenient use by the designer [3]. Dew point and bubble point compositions and the plate temperatures can be determined directly. Constant molal overflow is assumed, and relative volatility is held constant over sections of the column. 

Determine top tray temperature for use in relative volatility calculations by running a dew point on the overhead rapor. For total condenser its composition is same as distillate product. For a partial condenser, run a dew point on the column overhead vapor composition as determined by a material balance around the partial condenser, reflux, and product. 

Calculate ti and xj by dew point on vapor Vj. Then determine Hj, referring to top tray as number one in this case. 

Often, a reasonable and convenient way to understand the heat transfer process in a heat exchanger unit is to break down the types of heat transfer that must occur such as, vapor subcooling to dew point, condensation, and liquid subcooling. Each of these demands heat transfer of a different type, using different AT values, film coefficients, and fouling factors. This is illustrated in Figure 10-36. It is possible to properly determine a weighted overall temperature... 

CAUTION Enthalpy-Entropy charts apply only to the gaseous state, and if a gas is cooled below its dew point, condensation occurs and heat removal cannot be determined directly from the charts. 

Psychrometry has to do with the properties of the air-water vapor mixtures found in the atmosphere. Psychrometry tables, published by the US Weather Bureau, give detailed data about vapor pressure, relative humidity and dew point at the sea-level barometer of 30 in Hg, and at certain other barometric pressures. These tables are based on relative readings of dry bulb and wet bulb atmospheric temperatures as determined simultaneously by a sling psychrometer. The dry bulb reads ambient temperature while the wet bulb reads a lower temperature influenced by evaporation from a wetted wick surrounding the bulb of a parallel thermometer. 

Figure 41.7 enables one to determine dew point at reduced pressure. The left scale shows the dew point at the elevated pressure. Drop from the intersection of this value and the elevated pressure line to the reduced pressure line and then back to the left to read the dew point at the reduced pressure. 

In the previous discussion it has been assumed that the vapour is a pure material, such as steam or organic vapour. If it contains a proportion of non-condensable gas and is cooled below its dew point, a layer of condensate is formed on the surface with a mixture of non-condensable gas and vapour above it. The heat flow from the vapour to the surface then takes place in two ways. Firstly, sensible heat is passed to the surface because of the temperature difference. Secondly, since the concentration of vapour in the main stream is greater than that in the gas film at the condensate surface, vapour molecules diffuse to the surface and condense there, giving up their latent heat. The actual rate of condensation is then determined by the combination of these two effects, and its calculation requires a knowledge of mass transfer by diffusion, as discussed in Chapter 10. 

C05-0037. A weather report gives the current temperature as 18 °C and sets the dew point at 10 °C. Using data from Table 5A, determine the partial pressure of water vapor in the atmosphere and calculate the relative humidity. 

At a pressure of 10 bar, determine the bubble and dew point of a mixture of hydrocarbons, composition, mol per cent n-butane 21, n-pentane 48, n-hexane 31. The equilibrium K factors can be estimated using the De Priester charts in Chapter 8. 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

Solution To determine the location of the azeotrope for a specified pressure, the liquid composition has to be varied and a bubble-point calculation performed at each liquid composition until a composition is identified, whereby X = y,-. Alternatively, the vapor composition could be varied and a dew-point calculation performed at each vapor composition. Either way, this requires iteration. Figure 4.5 shows the x—y diagram for the 2-propanol-water system. This was obtained by carrying out a bubble-point calculation at different values of the liquid composition. The point where the x—y plot crosses the diagonal line gives the azeotropic composition. A more direct search for the azeotropic composition can be carried out for such a binary system in a spreadsheet by varying T and x simultaneously and by solving the objective function (see Section 3.9) ... 

The vapor-liquid x-y diagram in Figures 4.6c and d can be calculated by setting a liquid composition and calculating the corresponding vapor composition in a bubble point calculation. Alternatively, vapor composition can be set and the liquid composition determined by a dew point calculation. If the mixture forms two-liquid phases, the vapor-liquid equilibrium calculation predicts a maximum in the x-y diagram, as shown in Figures 4.6c and d. Note that such a maximum cannot appear with the Wilson equation. 

If a partial condenser is to be used and a vapor top product taken, then the above criteria should be applied to the dew point of the vapor top product, rather than the bubble point of the liquid top product. Also, if a vapor top product is to be taken, then the operating pressure of the destination for the product might determine the column pressure (e.g. overhead top product being sent to the fuel gas system). There are two major exceptions to these guidelines ... 

Experimental Determination of Enthalpy Departures of Well Defined Simulated Natural Gas/Water Mixtures." In addition, densities and dew points are being measured. 

Combined physical and chemical equilibrium. Vapour-liquid equilibria were determined in this work by performing dew-point calculations. The procedure is ... 

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